(1) Field of the Invention
The present invention relates to an image coding apparatus and an image coding method for coding interlace video signals, a recording medium storing a program causing a computer to execute the image coding method, and an integrated circuit corresponding to the image coding apparatus.
(2) Description of the Related Art
Recently, the multi-media era has come in which sound, pictures and other pixel values are integrated into one media, and conventional information media as communication tools like newspapers, magazines, TV, radio and telephone are regarded as the targets of multi-media. Generally, multi-media is a form of simultaneous representation of not only characters but also graphics, sound, and especially pictures. In order to handle the above-described conventional information media as multi-media, it is a requisite to represent the information digitally.
However, it is unrealistic to directly process a huge amount of information digitally using the above-described conventional information media because, when calculating the data amount of each information medium described above as digital data amount, data amount per character is 1 to 2 bytes while that of sound per second is not less than 64 K bits (telephone speech quality) and that of moving pictures per second is not less than 100 M bits (present TV receiving quality). For example, a TV telephone has already become commercially practical thanks to Integrated Services Digital Network (ISDN) with a transmission speed of 64 kbps to 1.5 Mbps, but it is impossible to transmit moving pictures of TV camera as they are using ISDN.
That is why information compression technique is necessary. For example, a moving picture compression technique standard of H. 261 or H. 263 which is recommended by the International Telecommunication Union-Telecommunication Standardization Sector (ITU-T) is used for TV telephones. In addition, with the information compression technique of the MPEG-1 standard, it becomes possible to store image information, together with sound information, in a normal CD (Compact disc) for music.
Here, Moving Picture Experts Group (MPEG) is an international standard to digitally compress moving picture signals, and has been standardized by the ISO/IEC (the International Standardization Organization/International Engineering Consortium). MPEG-1 is the standard to compress moving picture signals down to 1.5 Mbps, that is, to compress TV signal information to about one hundredth. Also, the quality which satisfies the MPEG-1 standard is medium level which can be realized at a transmission rate of about 1.5 Mbps. MPEG-2 is thus standardized in order to satisfy the need for higher picture quality (See Non-patent Reference). MPEG-2 compresses moving picture signals to 2 to 15 Mbps to achieve TV broadcasting quality.
At present, the work group (ISO/IEC JTC1/SC29/WG11), which standardized MPEG-1 and MPEG-2, has standardized MPEG-4 with new functions necessary in this multi-media era. The MPEG-4 standard achieves a compression rate higher than those of MPEG-1 standard and MPEG-2 standard, and enables coding, decoding and performing operations on an object-by-object basis.
The initial object of MPEG-4 standard is to standardize a coding method of pictures with low bit rates, but the object is extended to a general purpose coding method for video including interlace pictures with high bit rates. At present, ISO/IEC and ITU-T, in combination, has standardized MPEG-4 AVC (Advanced Video Coding) as a picture coding method for pictures with a higher compression rate.
Here, an image signal can be considered as a sequence of pictures (that are also referred to as frames or fields) each of which is a set of pixels having the same time. A current pixel has a high correlation with neighbouring pixels within a current picture, and thus the correlation between the pixels in the picture is used in the compression of an image signal. Consecutive pictures have high pixel correlation with each other, and thus the pixel correlation between the pictures is used in the compression of an image signal. Here, compression using (i) correlation between pixels in a picture and pixels in another picture, and (ii) correlation between pixels within a picture is referred to as inter coding or inter picture coding. On the other hand, compression using correlation between pixels within a picture without using correlation between pixels in a picture and pixels in another picture is referred to as intra coding or intra picture coding. With the use of correlation between pictures, inter coding makes it possible to achieve a compression rate higher than a compression rate obtainable in intra coding.
In MPEG-1, MPEG-2, MPEG-4, MPEG-4 AVC, and H. 264, intra coding and inter coding are switched in units of a block (or a macroblock) which is a set of two-dimensionally arranged pixels each having a rectangular area.
This is described in more detail. In the case of an intra coding, the image coding apparatus generates a predictive error block using correlation between pixel blocks within a picture or correlation between pixels within the picture. In the case of an inter coding, the image coding apparatus generates a predictive error block using correlation between pictures. Next, the image coding apparatus performs two-dimensional orthogonal transform such as DCT (Discrete Cosine Transform) on these blocks. Next, the image coding apparatus quantizes the frequency components of the blocks that have been subjected to the orthogonal transform, performs variable length coding on the quantized values obtained as the result of the quantization, and transmits the results of variable length coding to a network.
Here, when a quantization step size which is a quantization width used in quantization is large, a high compression rate can be obtained with a large degree of image coding distortion. In contrast, the use of a small quantization step size enables reduction in the degree of image coding distortion, resulting in a low compression rate. In general, the image coding apparatus derives quantization step sizes such that bit rates fall within a certain range and that the quantization step sizes within each picture become approximately constant. This is because uneven quantization step sizes within a picture causes uneven coding distortion, resulting in noticeable coding distortion.
On the other hand, high-speed network environments using ADSL or optical fibers have been widespread. With these environments, data can be communicated at a bit rate of several Mbps at home. Further, it is likely that data can be communicated at a bit rate of several tens of Mbps in the future several years. It is predicted that the use of the aforementioned image coding technique facilitates introduction of TV telephones and TV conference systems of TV broadcasting quality or HDTV broadcasting quality to not only corporations using exclusive lines but also general households.
When coded image data that is a stream is transmitted via a network, the stream may be partly lost due to network congestion or the like. In the case where the stream is partly lost, a receiving side cannot correctly decode the image corresponding to the lost part. In other words, the image quality deteriorates. To prevent this, a coding unit which is a set of blocks is defined as a slice. A slice is the minimum unit of independent coding and decoding. In other words, even when a part of a stream is lost, decoding can be performed on a slice basis.
FIG. 21 is a diagram showing the relationship between slices and blocks in the case of using a slicing method according to the MPEG-2 standard. A picture 500 (a frame or an I-field) shown in FIG. 21 is made up of plural blocks 502 (macroblocks). The blocks 502 in a line among the blocks 502 constituting the picture 500 make up of a slice 501.
FIG. 22 is a diagram showing the coding order of the blocks 502 included in the picture 500. The blocks 502 included in the picture 500 are coded according to the order shown in FIG. 22, that is, in the order from left to right within each slice and from the upper-most slice to the lower-most slice within the picture 500.
Here, in the case where the stream is partly lost as mentioned above, the receiving side cannot correctly decode the image corresponding to the lost part. Further, in the case where inter coding is used, the status where correct decoding is impossible affects (continues to) the following pictures to be decoded. Hereinafter, this is also referred to as error propagation.
An example of a known method for preventing such error propagation is the technique disclosed in the Patent Reference indicated below.
FIG. 23 is a diagram showing an exemplary structure of a stream used in the technique disclosed in the Patent Reference. As shown in FIG. 23, in coding processing performed by the image coding apparatus disclosed in the Patent Reference, an I-slice 201 (refresh slice) which is coded using only intra coding is included in each of pictures, and the position of a current I-slice 201 is sequentially moved on a picture-by-picture basis, and such movement is repeated for each refresh cycle. With this, the image coding apparatus disclosed in the Patent Reference can forcedly refresh the slices located at all the positions in a picture at least once during a refresh cycle, and therefore can prevent error propagation.